SECTIONS OP EXOGENOUS STEMS. 



81 



Figs. 134, 135, 136, 137, 138 exhibit horizontal and perpendicular sections of an 

 exogenous stem, from the end of the first to the end of the fifth year. In each figure 



Fig. 134. End of first year's growth 



III" ! I , ' 

 Fig. 135. End of second year's growth. 



Ill 



Fig. 136. End of third year's growth. 



Fig. 137. End of fourth year's growth. 



. , 



ii ! 



Fig. 138. End of fifth year's growth. 



the pith occupies the 

 centre, and is the largest 

 at the end of the second 

 year; after which it pro- 

 gressively diminishes. 

 Immediately around it is 

 the medullary sheath. 

 The bark is on the outer 

 boundary ; and the woody 

 and pitted tissues occupy 

 the intervening spaces, 

 and increase at the rate of 

 one layer or zone per year. The medullary rays pass from the pith to the bark. 



From the preceding remarks it will at once be inferred that a plant of one year's 

 growth has but one layer of wood ; and that that, therefon-, does not inclose wood, but 

 pith only. When the tree has reached the end of the seccnd season it will have two 

 layers ; and so on, successively, through any number of years. The above Figures 

 represent each a horizontal and vertical section of a steni at various periods ; and in 

 Fig. 138 it will be seen that the stem of a plant five years old exhibits a centra] 

 pith, five zones of wood, and the bark, besides the cambi/im in the spring season and 

 the medullary rays. 



The age of trees has been inferred, when a section of the whole stem could be 

 examined, by counting the number of rings of wood whit, h have been deposited around 

 the pith. When only a part of the stem remained, and yet its original diameter was 

 known, the same end has been s ught by multiplying the width of one zone by one half 

 the diameter, or by counting; the number of zones from the pith to the bark, should so 



